Nuclear reactors, for example boiling water and pressurized water reactors, pass water through a reactor core which contains nuclear fuel. The passing of this water through the reactor core heats the water. The water is heated to either a hot liquid phase (pressurized water) or a combination of a hot liquid phase and a vapor phase (boiling water). The water and/or steam are transported through systems in the nuclear power plant, such as the reactor pressure vessel, steam separators, pressurizers and steam generators to transfer the heat energy generated by the nuclear reaction to other working systems. These piping systems and components transporting the fluid are made of various materials which may be susceptible to corrosion and irradiation induced or assisted stress corrosion cracking.
Electrochemical corrosion potential (“ECP”) provides a guide to determining an amount of a oxidation/reduction reaction which occurs on a metal surface, for example on the surface of primary water coolant pipes. The oxidation/reduction reactions may depend, for example, on a dissolved oxygen concentration of water in a nuclear reactor, hydrogen concentration and/or hydrogen peroxide concentration obtained during water radiolysis. To decrease the electrochemical corrosion potential of these reactor coolant systems, the dissolved oxygen, and hydrogen peroxide concentrations of the water are kept as low as possible, preferably, to a level of about 25 parts per billion. This is performed, for example, by adding hydrogen to the system. Practically, however, maintaining dissolved oxygen, hydrogen and hydrogen peroxide concentrations at this low level is extremely difficult due to the changing water chemistry in the reactor coolant system.
Electrochemical corrosion potential measurements are made in nuclear power stations to determine whether corrosive conditions are occurring in the station and whether stress corrosion cracking is likely to occur. In particular, if the electrochemical corrosion potential value is relatively low (i.e. below a threshold value), corrosion rate and/or stress corrosion crack growth rates are not significant. Above the threshold value, however, the possibility of stress corrosion cracking and/or the corrosion rate increases when electrochemical corrosion potential values increase. Measurements of electrochemical corrosion potential are made at a single point in the primary coolant system on the materials of interest such as in the weakest materials of internals. Existing electrochemical potential probes contain sensors that are typically a metal to metal oxide configuration which respond to oxygen concentrations in the reactor water.
Existing systems used to measure electrochemical corrosion potential have many drawbacks. First, the probes used are fragile and are only operable for approximately three months as the sensors within the probes deteriorate from heat and radiation. As a consequence, the probes can only measure the electrochemical corrosion potential for less than 25% of the resident reactor core time precluding their usage around a nuclear reactor. Nuclear power plant operators' alternatives to alleviate this drawback are few. The nuclear power plant may be operated without monitoring corrosive conditions, however if the electrochemical corrosion potential is not measured for the entire fuel cycle, conditions may favor the formation of corrosion or stress corrosion cracking, thereby potentially damaging sensitive and expensive nuclear power plant systems. Alternatively, the nuclear reactor may be shut down and the electrochemical corrosion potential probes around the reactor are replaced. This alternative is economically unattractive due to the economics of a facility closure. The second drawback is that existing systems use a discrete measurement point probe for analysis. This type of system merely provides a spot measurement on an individual system. Existing systems cannot ascertain if the electrochemical corrosion potential is elevated in a part of the nuclear plant system not directly measured. The complex and changing materials through a nuclear power plant coolant system do not allow current systems to accurately measure electrochemical corrosion potential of systems relative to one another. As a consequence, certain systems or subsystems of the nuclear reactor are more prone to corrosion and stress corrosion cracking, as compared to others. Current systems do not allow the nuclear plant operator to compare data derived from measuring different systems, therefore attention is focused on the probe location. A true risk assessment analysis of the entire nuclear plant system is not performed. Current systems also do not determine an electrochemical corrosion potential for the zirconium clad fuel elements, as compared with the electrochemical corrosion potential measured for structural internals or piping materials. To date, current systems are limited to determining electrochemical corrosion potential of structural or piping members inside the reactor cooling systems.
There is a need to provide an electrochemical corrosion potential measuring system that will allow for a determination of an electrochemical corrosion potential of the zirconium fuel rods during an entire fuel cycle of a nuclear power plant.
There is a further need to provide an electrochemical corrosion potential measuring system that allows for replacement of a probe and its associated sensors at the end of its service life in a cost efficient manner.
There is also a need to provide an electrochemical corrosion potential measuring system that will determine the electrochemical corrosion potential of various materials (which make up the nuclear plant system) at the same time to provide data to a nuclear plant operator as to which nuclear systems are at risk for corrosion relative to other nuclear systems.
There is also a further need to provide an electrochemical corrosion potential measuring system that may be utilized to determine the amount of potential degradation of fuel rods during reactor operating conditions.